Description of issue:
When using the foreman api v1 to create a host, and ommiting the medium_id, theforeman proxy fails immediatly and silenty without logging in proxy.log that I haven't told it about a medium_id.

Specs:
Foreman 1.4.2 installed via foreman puppet modules
centos 6.4 x64
Foreman-proxy: DHCP and TFTP enabled, everything else disabled ( proxy installed via puppet modules )

How to reproduce:
- install foreman 1.4.2
- install foreman-proxy and enable just DHCP and TFTP
- create a new host via the API and don't supply the medium_id, supply just these parameters(adjusting the values for your setup):

architecture_id: 1, operatingsystem_id: '1', enabled: true, build: true, domain_id: 1, environment_id: '1', ptable_id: 6, subnet_id: 1

Output of logs:
- from foreman
- from foreman-proxy
- output via HTTP body from the API
you'll find all 3 in this gist https://gist.github.com/stefancocora/10369030

After ohadlevy advised over irc to add medium_id the TFTP build process started and completed successfuly.